A report in 1980 that LiCoO2 is useful for a cathode active material of lithium rechargeable batteries was followed by a lot of research, so LiCoO2 was adopted by commercial enterprises as a cathode active material for lithium rechargeable batteries. But, the high cost of LiCoO2 contributes significantly (about 25%) to the cost of the battery product. High competition presses producers of rechargeable lithium batteries to lower the cost.
The high cost of LiCoO2 is caused by two reasons: First, the high raw material cost of cobalt, and second the high cost of establishing reliable quality management and ensuring perfect process control during large scale production.
Especially, the quality management and process control aim to achieve highly reproducible products having optimized properties, where the performance of every batch fluctuates very little from those optimum properties. High reproducibility and little fluctuations of the performance of LiCoO2 are absolutely essential in current highly-automated high volume lithium battery production lines.
A major problem is that LiCoO2 is a sensitive material. Small changes of production process parameters cause large fluctuations of the performance of the cathode product. So quality management and process control require much effort and high costs.
LiCoO2 is a stoichiometric phase. Under normal conditions (for example 800° C. in air) no reliable indication for any Li:Co non-stoichiometry has been reported in the literature.
Only stoichiometric LiCoO2 with a Li:Co ratio very near to 1:1 has properties which are suitable for the cathode active material of the commercial lithium batteries. If the Li content is higher than 1:1, LiCoO2 will coexist with a secondary phase which contains the excess lithium and largely consists of Li2CO3. Li2CO3 impurities in the commercial LiCoO2 cathode active material are highly undesirable. Such samples are known to show poor storage properties at elevated temperature and voltage. One typical test to measure the storage properties is storage of fully charged batteries at 90° C. for 5 hours.
If the cathode contains Li2CO3 impurities, this may result in strong swelling (increase of thickness) of polymer cells. Even the much stronger metal cases of prismatic cells may bulge.
If the Li content is lower than 1:1, then the cathode contains divalent cobalt, i.e. LiCoO2 coexists with cobalt oxides. Lithium-deficient LiCoO2 shows poor cycling stability at a high voltage (>4.3 V), especially at an elevated temperature It is speculated that the higher catalytic activity of divalent cobalt present in the cobalt oxide phase supports the undesired oxidation of an electrolyte on the surface of LiCoO2. Alternatively, divalent cobalt might, especially at a high voltage, dissolve in the electrolyte, and undergo precipitation at the anode side, thereby damaging a solid electrolyte interphase (SEI) layer on the anode.
Only in a lab, it is easy to prepare stoichiometric LiCoO2 practically free of Li2CO3 or CoOx impurities by simple heating of LiCoO2. The high cycling stability of such cathodes (in coin cells) has been demonstrated in the literature. It is speculated that the good cycling stability is attributed to two effects: (1) At small scale (lab size samples) the excess lithium (Li2CO3) easily evaporates during sintering, and (2) Heating repairs any damage to the surface of LiCoO2, which was caused by air exposure, probably by a reductive attack by hydrocarbons.
A similar re-heating of LiCoO2 is not effective to solve the problems associated with the high temperature properties and cycling stability which may occur in the large scale production. First, large scale-produced LiCoO2 has not a damaged surface. After production the product is usually filled into air tight packaging, so any damage caused by air exposure is practically absent. Second, on a large scale, excess lithium does not evaporate practically. Very small amounts of Li2CO3 can be decomposed because volatile phases exist with very small thermodynamic equilibrium partial pressure. At small partial pressure gas transport is very slow, so that only tiny amounts of Li2CO3 can be decomposed. If we deal with large samples then the gas transport is not sufficient to decompose significant amounts of Li2CO3.
The situation is different if Li2CO3 decomposes in the presence of a lithium acceptor (such as cobalt oxide). In this case the thermodynamic equilibrium partial pressure is high and the gas transport kinetics is fast enough to decompose Li2CO3.
More generally, it is very difficult or even impossible to prepare LiCoO2 with the exact desired Li:Co ratio at large scale. If an excess of cobalt is used, then a cobalt oxide impurity remains. Unfortunately, small impurities of CoOx are practically impossible to be detected by standard quality control methods, but they are very important for the performance of the cathode. If an excess of lithium is used, lithium impurities remain due to the low evaporation at large scale. Even if the premixed (Li2CO3 and Co-oxide) powder would exactly have the desired Li:Co ratio, any inhomogeneity in the mixed powder would after sintering creates a powder with regions being lithium-rich and other regions being lithium-deficient. Additionally some Li2CO3 can melt before fully reacting with the Co-oxide, and the molten Li2CO3 would tend to separate downwards. This will cause a Li:Co gradient with Li-deficient sample at the top and Li-excess at the bottom of the sintering vessel. As a result, very small amounts of impurity phases (Li2CO3 or Co-oxide) are present.
Much previous art to improve properties of LiCoO2 has been disclosed. Examples of such efforts are surface coating of LiCoO2, doping of LiCoO2 with other metal cations and the preparation of non-stoichiometric LiCoO2 at a very high temperature. Each effort created some satisfactory results, but the results are not enough for a mass production process, and make another problem of the costs of additional processes.